Elsevier

Materials Research Bulletin

Volume 47, Issue 12, December 2012, Pages 4181-4186
Materials Research Bulletin

Investigation about thermophysical properties of Ln2Ce2O7 (Ln = Sm, Er and Yb) oxides for thermal barrier coatings

https://doi.org/10.1016/j.materresbull.2012.08.074Get rights and content

Abstract

Three kinds of rare earth cerium oxides – Ln2Ce2O7 (Ln = Sm, Er and Yb) were prepared by solid state reaction method. Their phase compositions, microstructures and thermophysical properties were investigated. Results of X-ray diffraction reveal that pure Ln2Ce2O7 (Ln = Sm, Er and Yb) oxides with fluorite structure are successfully synthesized in the current research. Scanning electrical microscopy results show that their microstructures are dense and no other un-reacted oxides or inter-phases exist in the interfaces between grains. Their thermal expansion coefficients are higher than those of YSZ, while their thermal conductivities are lower than those of YSZ. The decreasing ionic radius from Sm3+ to Yb3+ results in the descending thermal expansion coefficients from Sm2Ce2O7 to Yb2Ce2O7. The effective phonon scattering by atomic weight difference contributes to the decreasing thermal conductivities from Sm2Ce2O7 to Yb2Ce2O7. These results imply that synthesized rare earth cerium oxides have potentials to be used as novel candidate materials for thermal barrier coatings in the future.

Highlights

► We successfully prepared three types of new rare earth cerium oxides. ► We measured their thermophysical properties. ► These new ceramics can be explored as candidate ceramics for thermal barrier coatings.

Introduction

Thermal barrier coating system (TBCs) prepared by plasma-spraying method or electron beam physical vapor deposition technology can be used to protect high-temperature metallic components in advanced turbine engines, which can increase the inlet temperature with a consequent improvement of the efficiency or reduce the requirements for the cooling system [1], [2], [3], [4]. Currently, yttria stabilized zirconia (YSZ), especially zirconia containing 8 wt% yttria coatings is still the mostly used TBCs on rotating parts in gas turbines [2]. However, the major disadvantage of the YSZ is the limited operating temperature of 1473 K for the long term application [3]. At high temperatures, the t′-tetragonal phase transforms into the tetragonal and the cubic (t + c) phases. During cooling, the t-phase will further transform into the monoclinic (m) phase, leading to the volume increase and resulting in the formation of cracks in the coating [5]. Moreover, thermal expansion mismatch between the YSZ coating and metallic substrate caused by the low thermal expansion coefficients of YSZ (10 × 10−6 K−1, 16 × 10−6 K−1 for the Ni-based super-alloy) has been generally accepted as one of the strongest factors for coating failure [6].

To overcome the disadvantageous properties of YSZ and further increase the operating efficiency, it is urgently needed to develop new thermal barrier oxides with a significantly lower thermal conductivity than YSZ.

The selection of TBC materials is restricted by some basic requirements, such as high melting point, no phase transition from ambient temperature to service temperature, low thermal conductivity, chemical inertness, relative high thermal expansion coefficient, good thermal shock resistance and low sintering ability [3], [4]. However, it is very difficult to find a few of ceramic materials which can meet to all requirements at one time, and many researchers always regard lower thermal conductivity, higher thermal expansion coefficient as the chief requirements of new candidate ceramic materials for TBCs [5], [6], [7]. According to the current research status, the reported candidate ceramic materials for TBCs can be classified into three groups, the first is the co-doped yttria-stabilized zirconia (YSZ) with one or more rare earth oxides, the second is rare-earth zirconates of A2B2O7-type, and the third can be named as other new-ceramic materials, such as rare earth doped CeO2 or HfO2–Ln2Ce2O7 [8], [9], [10], [11], [12], [13] or Ln2O3–HfO2 [14], [15], LnInFeZnO4 [16], BaLa2Ti3O10 [17] and LaTi2Al9O19 [18].

Thermophysical properties of YSZ can be improved by co-doping with one or rare earth oxides, however, there still exists phase transformation if their working time is too long at temperature above 1500 °C, which limits their applications for new thermal barrier coatings.

In recent years, the rare earth zirconates with general formula Ln2Zr2O7 (Ln = rare earth elements) with pyrochlore structure or defect fluorite-type structure have been regarded as the most potential ceramic materials for thermal barrier coatings. The thermal conductivities of Ln2Zr2O7 (Ln = La, Nd, Sm, Eu, Gd, Dy, etc.) ceramic materials are in the range from 1.1 to 1.2 W/m K, which are much lower than those of YSZ. Because of their promising thermophysical properties, efforts have been made to investigate the co-doped Ln2Zr2O7 ceramics with one or more metal oxides in recent years [9], [10], [11], [12], [13]. However, their relative low thermal expansion coefficients (TCEs) can result in high thermal stresses in TBC applications, which is very harmful for performance of TBCs.

In the third group, rare earth doped CeO2 oxides have attracted extensive attention due to their excellent electrical, catalytic, mechanical properties, low thermal conductivities and high thermal expansion coefficients at high temperature. Thermophysical properties of several interesting rare earth cerium oxides, such as La2Ce2O7 [8], Nd2Ce2O7 [9], Gd2Ce2O7 [10], [11], Y2Ce2O7 [12] and Dy2Ce2O7 [13], have been investigated. Results indicate that these rare earth cerium oxides have potential to be used as new candidate materials for future TBCs. However, thermophysical properties of a few rare earth cerium oxides, such as Sm2Ce2O7, Er2Ce2O7 and Yb2Ce2O7, have not been reported up to now. Therefore, Ln2Ce2O7 (Ln = Sm, Er and Yb) ceramic materials were synthesized by solid reaction, and their microstructure and thermophysical properties were examined in current work.

Section snippets

Experiment

Ln2Ce2O7 (Ln = Sm, Er and Yb) oxides were prepared for this study using CeO2 (purity  99.99), Sm2O3 (purity  99.99%), Er2O3 (purity  99.99%) and Yb2O3 (purity  99.99%) powders. The samples were prepared by solid-state reaction. After weighing, the powders were mixed by planetary milling with zirconia balls in isopropyl alcohol for 6 h. The dried powder mixtures were then sieved for granulation and compacted into a disk form under uniaxial pressure of 50 MPa followed by cold isostatic pressing with 150 

XRD

The XRD patterns of these synthesized ceramic materials are shown in Fig. 1 together with those of standard CeO2. Fig. 1 indicates that their XRD patterns are coincident with standard spectrum of CeO2, which means that pure rare earth cerium oxides Ln2Ce2O7 (Ln = Sm, Er and Yb) with defect fluorite structure are successfully synthesized in the current research. It is well known that there are two kinds of crystal structures of A2B2O7 oxides, which are pyrochlore structure and fluorite structure.

Conclusions

  • (1)

    Single-phase Sm2Ce2O7, Er2Ce2O7 and Yb2Ce2O7 ceramic materials with fluorite crystal structure were synthesized successfully by solid reaction method at 1600 °C for 10 h using Sm2O3, Er2O3, Yb2O3 and CeO2 as the raw materials.

  • (2)

    Their thermal expansion coefficients increase with increasing temperature from 20 °C to 1000 °C. The technical thermal expansion coefficients of Sm2Ce2O7, Er2Ce2O7 and Yb2Ce2O7 at 1000 °C are in the range 11.2 × 10−6 to 11.8 × 10−6 K−1, which are higher than those of YSZ.

  • (3)

    Their

Acknowledgment

This work was supported by Doctor Research Fund (D2007012) in Henan Institute of Engineering.

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